1. Field of the Invention
The present invention relates generally to laterally translatable seal carrier mechanisms having rotary shaft seals. More particularly, the present invention is directed to an axially hydraulic force balanced laterally translatable seal carrier mechanism having a pressure staging system which divides a fluid pressure across two or more rotary shaft seals and also provides for active seal cooling and bearing cooling. In several embodiments, the axially hydraulic force balanced laterally translatable seal carrier system is substantially immune to pressure breathing. The invention also provides for enhanced rotary seal extrusion resistance, high operating pressure capability, and compatibility with rotary seal hydrodynamic lubrication.
2. Description of the Prior Art
A particularly difficult sealing situation occurs when a housing containing high pressure fluid is penetrated by a relatively rotatable shaft of large diameter. The difficulty is compounded when available axial space for the shaft sealing mechanism is limited. The energy industry, in particular, is currently in need of a compact high pressure rotary shaft sealing mechanism for sealing pressures in the range of 3,000-10,000 psi on shafts of approximately 8-24 inches in diameter which are subjected to severe mechanical loads and large resulting deflections.
Resilient rotary seals require a very small shaft to housing clearance and minimum runout for optimum high pressure extrusion resistance. If the shaft to housing clearance is too large, the pressure causes the seal to bulge or protrude into the shaft to housing clearance and suffer classic extrusion damage leading to early failure. The protrusion is a direct function of the pressure, the seal modulus of elasticity, and the size of the shaft to housing clearance. The larger the clearance, the more the protrusion. The inevitable dynamic runout of a large diameter shaft, in conjunction with "pressure breathing" of a large housing, efficiently destroys seal material which protrudes into the shaft to housing clearance. Cyclic strain causes the protruding material to break away from the seal, only to be replaced by more protruding material until the seal ultimately fails.
In practice, the shaft to housing clearance must be as small as possible to avoid seal extrusion damage, but it must also be sufficiently large to avoid shaft to housing contact. If shaft to housing contact occurs, the housing assumes part of the side load intended for other bearing locations, and the resulting friction and heat can damage the shaft and housing and destroy the rotary seal. Unfortunately, if the shaft to housing clearance for a large shaft is in a useful range for resilient rotary seals, it is virtually impossible to guide the shaft so precisely that it will not rub on the housing bore. It is therefore desirable to provide a laterally translatable rotary shaft sealing mechanism which provides efficient sealing at high operating pressures while minimizing the potential for seal extrusion and heat induced seal failure.
A number of factors prohibit a small shaft to housing clearance and precise shaft guidance in large machinery. Manufacturing tolerances are large, which directly affects the size of shaft to housing clearance, and also increases bearing mounting clearance and bearing internal clearance, which permits shaft runout and misalignment. Large components are often subject to significant elastic deformation when exposed to high pressure and large mechanical loads, and are often subject to significant dimensional variability from differential thermal expansion and contraction caused by seal and bearing heat. Such dimensional variability can have a dramatic effect on assembly clearances and bearing internal clearances, which can permit large dynamic runout and lateral offset of the shaft, and which can also cause large variations in shaft to housing clearance. It is desirable therefore to provide an efficient laterally translatable rotary shaft sealing mechanism which may be utilized effectively for large shaft diameters as well as high pressure conditions and is capable of accommodating conditions of enhanced dynamic runout and housing pressure breathing while effectively minimizing pressure responsive seal extrusion and thereby providing for extensive service life of the rotary shaft seals thereof.